Boys are typically more severely affected than girls. While most boys have mental retardation, only one-third to one-half of girls have significant intellectual impairment; the rest have either normal IQ or learning disabilities. Emotional and behavioural problems are common in both sexes.

What Causes Fragile X?

In 1991, scientists discovered the gene (called FMR1) that causes fragile X. In individuals who have fragile X syndrome, a defect in FMR1 (a "full mutation") shuts the gene down. Like a defective factory, FMR1 cannot manufacture the protein that it normally makes. Other individuals are carriers: they have a small defect in FMR1 (called a "permutation") but do not show symptoms of fragile X.

Fragile X is inherited. Carrier men (transmitting males) pass the permutation to all their daughters but none of their sons. Each child of a carrier woman has a 50% chance of inheriting the gene. The fragile X permutation can be passed silently down through generations in a family before a child is affected by the syndrome.

Genetic Cause of Fragile X

The FMR1 gene is located on the long arm of the X chromosome. Within this gene lies a region of DNA, which varies in length from one person to another. Ordinarily, this stretch of DNA falls within a range of length that would be considered "normal".

In some people, however, this stretch of DNA is somewhat longer; this gene change is called a "permutation". Although a person who carries the permutation does not typically have symptoms of fragile X, the stretch of DNA is prone to further expansion when it is passed from a woman to her children. When the stretch of DNA expands beyond a certain length, the gene is switched off and does not produce the protein that it is normally makes. This gene change is called a "full mutation".
A male who inherits a full mutation exhibits characteristics of fragile X syndrome because his only X chromosome contains the mutated gene. A female may not be as severely affected as a male because each cell of her body needs to use only one of its two X chromosomes and randomly inactivates the other